Liquid level gauge



Jan. 1, 1952 A. EDELMAN LIQUID LEVEL GAUGE 2 SI-IEETS-SHEET 1 Filed Feb.1, 1945 IN V EN T 0R. AER/MAM 52762401 BY Jan. 1, 1952 A. EDELMAN LIQUIDLEVEL GAUGE 2 SHEETSSHEET 2 Filed Feb. 1, 1945 INVENTOR. 554/144 MPatented Jan. 1, 1952 UNITED STATES PATENT OFFICE 2,ss1,oss mourn LEVELGAUGE Abraham Edelman, New York, N. Y., assignmto The LiquidometerCorporation, Long Island City, N. Y., a corporation of DelawareApplication February 1, 1945, Serial No. 575,664

16 Claims.

capacitance between the electrodes oi a trans-' mitter capacitor.

Telemetering devices of the type described may be used to measure thechanges of the fluid contents of a container. The fluid to be measured,for example a liquid, will occupy more or less of the electric fieldspace between the electrodes according to the amount of liquid present.I! now the medium displaced by the'liquid has a dielectric constantwhich is diilerent from that of the liquid to be measured, thecapacitance between the electrodes will vary with the level or amount ofliquid to be measured. Usually air is used as the medium to be displacedby the liquid to be measured, air having a dielectric constant diiierentfrom that of all liquids.

With devices of this type there is the difiiculty that the capacitancebetween the electrodes will be controlled not only by variations of thelevel or volume of the liquid but also by changes of the dielectricconstant of the liquid. As is well known, the dielectric constant of aliquid or any variations the dielectric constant of the liquid willchange, causing a corresponding change of the total capacitance of thecapacitor and hence aflecting the indications of the teiemeteringdevice.

It will be apparent from the previous explanation that variations of thedielectric constant will affect indications of the telemetering devicein response to changes of the level or volume of the fluid so that suchindications will not represent volume of the fluid.

Another object of the invention is to provide Another object of theinvention is to provide means for eliminating all errors in theindications one indicator.

Other and further objects, features and advantages of the invention willappear hereinafter and will be specifically pointed out in the appendedclaims.

It should be noted that the term and expression fluid" as herein usedshall and does include conducting and non-conducting liquids such asgaso-' designated as Ca; with liquid between the plates, as CF; withpart liquid and part air, as Cu, so that On varies from a minimum valueof CE to a maximum value of CF as the electric field space between themetal plates is varied from empty to full of the liquid in the tank. Theplates are asconstant, A the area, b a constant, and d the spacingbetween the plates.

The capacitance Cn may be considered as consisting of two condensers inparallel, one having a dielectric K1. and an area A1,, and the otherhaving a dielectric 1.0, and an area A-Ar. That i 4 If now a circuitsystem is provided for the condenser which furnishes a currentproportional to (Ca-Cs) such current, as the above formula shows, mustalso be proportional to (Kz.1). In

Kr... There are numerous bridge type circuits which (Kr. 1) and is,therefore, also independent of balance one condenser against another,thus providing a current which is proportional to the dif-.

ference between two capacitances. There are also other circuits whichbalance out part of-a'r-capaci tative impedance by employing inductors.

The present application is related in subject matter to the copendingapplication of Son-,

theimer, Serial Number 151,452, filed January 31, 1950, entitledMeasuring System with Compensation for Dielectric Constant, whichapplication is owned in common with the present application. These twoapplications have priority in the order of their respective filingdates; in other words, the present application is to carry all claimsgeneric to the two applications, while the Sontheimer application is tobe restricted to subject matter disclosed therein and not disclosed inthe present application. The lines of division between these two casesare based upon these principles.

In the accompanying drawings several embodiments of the invention areshown by way of illus Q trat'on and not by way of limitation.

In the drawings:

Fig. 1 shows a diagrammatic circuit arrangement according to theinvention providing 9. current which is proportional to the differencesbetween two capacitances;

Fig. 2 is a digarammatic circuit arrangement according to the inventionemploying inductors for balancing out part of a capacitive impedance;

Fig. 3 is a circuit arrangement similar in which oscillators are used;

Fig. 4 is a diagrammatic perspective view of a condenser assembly to beused in connection with circuit arrangements according to the invention;Y

Fig. 5 is a diagrammatic perspective view, partially broken away, of amodification of a condenser assembly to be used with circuitarrangements according to the invention;

Fig. 6 is a similar view of still another modiflcation of a condenserassembly to be used in circuit arrangements according to the invention;

and I Fig. '7 is a detail, on an check valve shown in Fig. 6.

Referring to Fig. 1, l is a source of A.-C., 2 is an input transformerhaving two similar centertapped secondaries 3 and 3'. Bridged across theouter terminals of secondary 3 are two condensers l and 5 connected inseries. Bridged across the two outer terminals of secondary 3' are twoother condensers l and 5', condensers l and 4' being variablecondensers. Condenser comprises several condenser units 5'1, 5'2, etc.all in parallel, and each immersed more or less in liquid within tanks15. Each condenser unit comprises an electrode is and a second electrodecomposed of two sections l1, l1.

In such circuit system the current in the branch between junctions 6 andI will be-proportional to the difierence between capacitance l andcaenlarged scale. of a to Fi 1' --which they are derived,

be proportional to the capacitance differences.

stood that the condenser pacltance 5; and similarly, the current in thebranch between junctions 6' and 1 will be proportional to the differencebetween capacitances l and 5'. These two currents which are, of course,A.-C. currents are passed through rectiflers 8 and 8', and then used toenergize coils 9 and 9' of a ratiometer with D.-C. current, which myIbTfiltered-by meansof iilt fiflndensers l0 and. ..'iB"E."-'"T he we.currents;will bet-approximately proportional to the A.-C. currents fromand therefore will also The 'ratiometer comprises a permanent-magnetrotor -|,l;which.supports a pointer l2 coactin .withza dial 13.calibrated, for example, in units of liquid-volume:

Condensers 5 and 5' include as dielectric a liq'i'lid'having the samedielectric constant K1. (the dielectric constant of the liquid beinggauged) while condensers 4 and 4' may have any dielectric independent ofthe. particular liquid being gauged. The values of capacitance ofcondensers 4 and t. are adjusted in advance to be equal to the values ofcapacitance in condensers 5 and 5' when the latter have air. as dielectric. With such an adjustment, no current will flow in the branches5-! and 51-1, and the ratiometer pointer 12 is uncontrolled. How; ever,this condition does not occur i i-practice. Condenser 5 alwayscontains-liquid as dielectric and after condenser 4. has been adjustedthere is always a diiierence in capacitance between con-Q densers 4 and5. In practice, when all the .units of condenser 5' are empty'of liquid,no current flows in the ratiometer. coil 91, and the permanent-magnetrotor H' of theratiometer is controlled only by coil 9. The u andpointer- I? so that the dial indication is at Empty.- If liquidentersany or allof the unitsof condenser 5', the total capacitance ofcondenser 5' is increased, and current flows through the coil 9 inproportion to the capacitance difference between condenser 5' and. l.The ratiometer pointer movement is then determined by both coils 9 and9'. When the liquid fully occupies the dielectric space in the units of.condenser .i', and thecurrent in coil 9' is at. its maximum value, thepointer l2 will indicate some other position on the dial l3, which maybe marked Fullfif g I If the liquid inboth condensers 5 and 5' nowbereplaced withsome other liquid having another. dielectric constant, thenthe same indications for fEmptyJf Full/{and for intermediate positionswill result since the currents in the coils Sand 9' will bear the sameratioto each other as before, differing fromtheir previous values. onlyby the common factor due to the changed dielectric constant.

In thecircuit arrangement shownin Fig. 2 the source i of A.-C. isemployed directly, without inter-posing a transformer, condensers 5 and5', are the same asin Fig. l,-but it should be under- 5' maybeconstitutedby several separatecondenser units in parallel as before.Inductors 4i and M are used in place of the condensers 4 and 4' shown inFig. 1. Theseinductors perform their functions by supplying torectifiers 8 and- 8' a current opposite in phase; to that supplied bycondensers 5 and 5'. Thus,- the total current into-rectifiers 8 and --8'is reduced in amplitude to any desired extent.

- Inductors t land: M areadjusted-so that the current, from condensersSand 5' isbalanced out. when air dielectric is used in them; A smallresidue of current may exist'in rectifiers 8 and-8,'but this has notbeen found to-be important in obtaining reasonably accurate indicationsof liquid level.

The rectified current' output from rectifiers 8 and 8' may be filteredby filter condensers l and I0, and is applied through ratiometer coils 9and 9'.

As previously explained, for the condition in which no liquid is presentin either condenser or 5, no current exists in ratiometer coils 9 and 9,and the pointer position is indeterminate. However, as mentioned inconnection with Fig. 1, in practice condenser 5 is always filled withliquid after inductor 4| has been properly adjusted. As a result, thereis sufilc'ient current in coil 9 to rotate the rotor into a definiteposition, and cause pointer I2 to indicate Empty on dial l3.Furthermore, if condenser 5' is now assumed to be filled gradually withliquid, the current in coil 9' will increase gradually also, and thiswill rotate the pointer until it points to Full at the time the liquidhas reached a desired level.

As will now be clear, a change in the dielectric constant of the liquid,as might occur if r the level measuring equipment is employed from timeto time for measuring difierent dilutions or difierent chemicals, willaffect both coils proportionally, and so will not influence the positionof the pointer on the dial.

Fig. 3 illustrates a third circuit arrangement in which the principlesof the invention are applied. Three conventional-type electronicoscillators I, l, l are supplied from a source of D.-C. |0|'. Eachoscillator includes in its resonating or tank circuit a condenser toground such as 5|, 5|", 5|"'. All three of these condensers are placedin one tank or container and correspond to any one of the condenserunits of Fig. 1 such as 5'1 or 5'2 and it will be understood that thenatural or resonant frequency of each oscillator will vary with thecapacitance value in each such condenser. However, the oscillators arecoupled together through coupling circuits that include leads l4, I4",|4"', rectifiers 8, 8", 8", and also ratiometer coils 9, 9", 9", towhich filter condensers l0, l0, l0" are respectively connected inparallel. Because of this coupling, the oscillators are unable tooperate at their resonant frequencies, but influence each other bysupplying signals through the coupling circuits. The interaction betweenthe oscillators is such as to cause them all to operate at one commonfrequency; The difierence between this common frequency of oscillation,and the natural frequency at which each oscillator tends to operatecauses the oscillators to be shifted in phase from each other inproportion to this difference. The phase diiferences betweencorresponding junctions 6, 6", 6 of the oscillator circuits thenestablish the magnitudes of the currents flowing through the couplingrectifiers and ratiometer coils.

It will now be obvious that in Fig. 3 each of the condensers 5|, 5|",5|" is compared with the other two, while in Fig. l condenser 5 iscompared with the total capacitance of transmitformed.

ter condenser 5', or in other words, both systems are based on the sameprinciple of comparing capacitances.

The condensers 5|, 5|, 5| are preferably combined to a single assemblyas shown in Fig. 4. The latter figure illustrates three plates 52, 52",52", each being an ungrounded electrode of one of the condensers 5|,5|", 5| As shown It has been found desirable to shape the outlines ofplates 52, 52", 52" approximately as shown in Fig. 4. The outlines ofthe plates are shaped and selected in such a manner that the Empty"capacitances of condensers 5|, 5|", 5| be all equal; and that as theliquid level changes, the capacitance differences -5|5|".

tinuously. The exact configuration of the plates can be convenientlyascertained by calculation and experiment.

Since the capacitances are all equal when air the dielectric, there isno phase shift between the three oscillators, and no current flows inthe coupling branches of the ratiometer coils, so that the pointer ofthe ratiometer will have no determinate position. To correct thissituation 2. preferably rectangular addition or extension 54 is providedat the bottom of plate 52", as shown in Fig. 4. That is, part of thearea of plate 52" has been placed below plates 52" and 52.

When the level of the liquid is below all the caused to move into adefinite Empty position by the use of a pointer control magnet I02 (Fig.3) or any other suitable means.

As the liquid level rises and reaches extension 54 of plate 52, butbefore it reaches plates 52 and 52", the current in some of theratiometer coils begins to flow in proportion, moving the pointer l2from the Empty position. During this period of measurement, however, theindications of the ratiometer are dependent on changes in the supplyvoltage, and are not accurate. However, when the liquid level reachesplates 52 and 52", the current created in the ratiometer coils issufiicient to furnish a rotor-controlling force in the ratiometer whichis much greater than that produced by the pointer control magnet I02, sothat the latter exerts an insignificant influence thereafter on thepointer position.

As the liquid level rises along the three plates, the ratiometer coilcurrents change continuously; and by suitably designing the shapes ofthe three plates in relation to each other, the pointer movement can bemade approximately uniform with level change. Furthermore, the pointertravel can now be made to encompass 360 or even several revolutions.

It will be also apparent that during the entire measurement from thebottom of plates 52 and 52" to Full, the conditions required forelimination of error due to dielectric constant have been fulfilled, andthat therefore the same level indications will result for widevariations in dielectric constant of the liquid. Furthermore,

part 54 may be in the sump of the tank, so that a negligible volume isassociated with the measurement of the inaccurate part of the range ofmeasurement.

Fig. 5 shows a liquid-filled condenser suitable for use as condenser 5in a circuit such as illustrated in Fig. 1. It includes a fillingaperture 42, a liquid-proof container 43, stationary capacitanceelectrodes 44, adjustable capacitance electrodes 45, a shaft 46 foradjustment of electrodes 4|, and terminals 41. The condenser may also beemployed in the circuit of Fig. 3 in place of the rectangular portion 54of plate 5 2" which is below the level of the other two plates. If

so employed, the ratiometei will never have zero current in all ofmagnet I02 may ly and the measurement from "Empty" to Full."

Fig. 6 shows another liquid-filled condenser suitable for use ascondenser 5 or in place of portion 54 of plate 52". This condenser isdesigned for incorporation into the tank. It includes a container 48,open at the bottom and having an air-check valve 49 at the top,terminals 50 and electrodes 55 'inside. Air-check valve 8, which may beof any suitable design, is shown in Fig. I as comprising a flap 60 and avalve seat 8| pivotally supporting the flap by means of a pivot 62. Thevalve seat may be mounted in a tubular member 63 fastened to container48. When the tank is filled with liquid, the flap 60 of air-check valve49 will be lifted by the pressure of the escaping air, thus permittingthe container to fill up, and to remain full thereafter until the liquidlevel is below the bottom of the container, the flap being pressedagainst its seat by air pressure.

If the liquid level falls below the bottom of this container, it emptiesof liquid, and thereafter the measurement is inaccurate. However, it isoften practical to bring the bottom of the container 48 down to nearlythe bottom of the tank, so that the tank is known to be practicallyempty when the liquid-filled condenser empties. Furthermore, since thiscreates the previously-described condition in which no current flows inam! of the ratiometer coils, it is possible to employ a pointer-controlmagnet such as I02 shown in Fig. 3, to bring the pointer to Empty or toan oil-scale position.

It should be understood, that the invention is not limited tonon-conducting liquids. It is equally practical to take measurements onconducting liquids, provided only that some of the electrodes are coatedwith an insulating coating, thereby preventing the passage of resistivecomponents of current in parallel with capacitative components. i

The invention is furthermore not limited to liquids, but may be used forthe measurement of powdered and granular materials, and semiliquids.Tanks in which the space above the liquid or material being gauged isoccupied by air have been previously described; but it should beunderstood that various fluids other than air, such as liquids, may bedisplaced by the material to be measured, the principles disclosed willstill apply without essential change.

The term ratiometer has been used in the foregoing to describe a form ofindicating instrument in which the indication does not vary if the ratiobetween the currents supplied to it remains unchanged. The discussionshould be understood to include'all such instruments, including thosewhich have moving coils or stationary coils; permanent magnet rotors orsoft iron rotors or coil type rotors; those which operate from D.-C. forA.-C.; also, such other electrical and electronic indicators as mayfurnish an indication representative of the ratio or difference betweencurrents, without regard to their magnitude. I

While the invention has been described in detail with respect to certainparticularly preferred be then dispensed with entireis independent ofK1.

its coils. The pointer controlexamples, it will be understood by thoseskilled in the art that various changes and modifications may be madewithout tions in the appended claims.

What is claimed is:

1. Apparatus for indicating the level of nongaseous fluids in acontainer adapted for use from time to time in indicating the levels ofdiflerent fluids, comprising an electric indicating instrument, at leasttwo electric circuits for controlling the indication of said instrument,said circuits being adapted to be energized by a single source ofalternating current. a condenser including spaced electrodessubstantially vertically arranged in said container, so that saidelectrodes will be immersed in the fluid in said container to an extentdependent upon the level of the fluid therein, whereby the capacitanceof said condenser will vary with the level of the fluid in saidcontainer, conductors connecting said condenser in one of said circuits,9. comparison condenser including electrodes. similarly connected in theother of said circuits and having its electrodes substantiallycompletely immersed in a fluid which has substantially the samedielectric values as the" fluid in the container throughout the range ofambient conditions to which the entire apparatus may be subjected duringoperation of the apparatus, said comparison condenser being positionedso as to be subject to the same ambient conditions as the flrst namedcondenser, said comparison condenser being adapted for change of itsassociated fluid to maintain the aforesaid relationship of thedielectric values whenever the first named condenser is to be used witha different fluid, and a variable reactance connected in each of saidcircuits to balance said circuits and to provide a zero, set-upadjustment for said indicating instrument.

2. Apparatus in accordance with claim 1, wherein said comparisoncondenser has its electrodes immersed in the same fluid. the level ofwhich is being measured by said first named condenser.

3. Apparatus in accordance with claim 1, wherein said comparisoncondenser is located within the same container as the fluid, the levelof which is being measured by said first named condenser, and issubjected to the same ambient conditions as said fluid.

4. Apparatus in accordance with claim 1, wherein said comparisoncondenser is located within the same container as the fluid, the levelof which is being measured by said first named condenser, and whereinsaid comparison condenser has its electrodes immersed in said fluid. soas to be subject at all times to the same ambient conditions as saidflrst named condenser.

5. Apparatus in accordance with claim 1, wherein said circuits aresimilar bridge circuits, and wherein the unbalance voltages of each ofsaid bridge circuits respectively are connected to control theindication of said instrument.

6. Apparatus in accordance with claim 1, wherein each of said circuitscomprises a bridge circuit, two arms of which are formed by acenter-tappedysecondary of a transformer, the primary of whichis'energiaed by said source of alterating current, there being two suchcentertapped secondaries of said transformer for said circuitsrespectively, wherein said variable reactance in each of said circuitsis connected in a third arm-oi each of said bridge circuits, and

departing from the spirit and scope of the invention, and it is intendedtherefore, to cover all such changes and modiflcawherein said firstnamed condenser and said comparison condenser are respectively connectedin the fourth arm of each of said bridge circuits, the unbalancevoltages of each of said bridge circuits being connected to control theindication of said instrument.

' 7. Apparatus in accordance with claim 1, wherein each of said circuitsis a bridge circuit, the unbalance voltages of each of said circuitsrespectively being connected to control the indication of saidinstrument, wherein said unbalance voltages are supplied to saidinstrument respectively through rectifiers, and wherein said instrumentis a direct current instrument having two deflecting coils energizedthrough said rectifiers from said circuits respectively.

8; Apparatus in accordance with claim 1, wherein in each said circuit,said variable reactance is an inductance connected in parallel with thecondenser in the respective circuits.

9. Apparatus in accordance with claim 1, wherein said two circuits areconnected in parallel with said source of alternating current, each ofsaid parallel circuits including said first named condenser and saidcomparison condenser respectively, wherein said variable reactance ineach said circuit is an inductive reactance connected in parallel withthe condenser in each of said circuits, a rectifier in each of saidcircuits connected in series with the condenser-reactance thereof acrosssaid source of alternating current, and wherein said indicatinginstrument is a direct current ratiometer-type instrument having twodeflecting coils arranged to be operated by direct current and energizedrespectively by the rectified currents from each of said circuits.

10. In a system for measuring liquid levels by the measurement ofcapacitance changes resulting from changes in the liquid level,apparatus for reducing errors caused by variation in the dielectricconstant of the liquid, comprising a measuring condenser having spacedplates immersed in the liquid, the level of which is to be measured toan extent dependent upon such level and arranged so that its capacitanceis a function of the level and dielectric constant of said liquid, acompensating condenser having spaced plates immersed in a representativesample of said liquid and arranged to be subject to the same ambientconditions affecting capacitance. so that its capacitance is a functionof the dielectric'constant of said liquid and is independent of thelevel which is being measured, first circuit means including saidmeasuring condenser and responsive to changes in the reactance thereof,

second circuit means including said compensating condenser andresponsive to changes in the "reactance thereof, alternating currentpower supply means energizing said circuit means, and

indicating means under the mutual control of said first and secondcircuit means.

11. In a system for measuring liquid levels by the measurement ofcapacitance changes resulting from changes in the liquid level,apparatus for reducing errors caused by variation in the dielectricconstant of the liquid, comprising a :measuring condenser having spacedplates ex tending into the liquid, the level of which is to be measured,to an extent dependent upon such level, so that its capacitance is afunction of the level and the dielectric constant of said liquid, acompensating condenser having spaced plates at all times completelyimmersed in a representative sample of said liquid which is subject tothe same ambient conditions affecting capacitance so that itscapacitance is a function 01' the dielectric constant of said liquid andindependent of the level which is being measured, first circuit meansincluding said measuring condenser and responsive to changes in thereactance thereof, second circuit means including said compensatingcondenser and responsive to changes in the reactance thereof,alternating current power supply means energizing said circuit means,and comparison means under the mutual control of said first and secondcircuit means and responsive to the relative impedances of said firstand second circuit means as a measure of the level of said liquid.

12. In a capacitance-type system for measuring liquid levels, apparatusfor reducing the error caused by variations in the dielectric constant,comprising an indicator, a circuit for controlling the indication ofsaid indicator, power supply means energizing said circuit, means forproducing an electrical indicator-actuating force that is a function ofthe level and dielectric constant of the liquid, the level of which isbeing measured, comprising a first electrical branch forming part ofsaid circuit and including a first condenser having spaced platesextending into said liquid and immersed therein to an extent dependentupon the level of the liquid, so that the capacitance of said condenseris a function of the level and the dielectric constant of the liquid,and means for modifying the value of said electrical indicator-actuatingforce in such direction as to reduce the effect thereon of varitaions inthe its capacitance is a function of the dielectric constant of saidliquid and independent of the level being measured.

13. Apparatus in accordance with claim 12, wherein said power supplymeans comprises a source of alternating voltage, and comprising inaddition, a first inductance connected in said circuit to said firstcondenser and having a reactance at the frequency of said alternatingvoltage equal to the magnitude of the reactance of said first condenserwhen free of said liquid, and a second inductance connected in saidcircuit to said second condenser and having a reactance at the frequencyof said alternating voltage equal to the magnitude of'the reactance ofsaid second condenser when free of said liquid.

14. Apparatus in accordance with claim 10, wherein said alternatingcurrent power supply means is a voltage source of constant frequency,and comprising in addition, a first inductance connected to saidmeasuring condenser and having a reactance at the frequency of saidalternating voltage equal to the magnitude of the reactance of saidmeasuring condenser when free of said liquid, and a second inductanceconnected to said compensating condenser and having a reactance at thefrequency of said alternating voltage equal to the magnitude of thereactance of said compensating condenser when free of said liquid.

15. Apparatus according to claim 10, in which said first circuit meanscomprises a first network including said measuring condenser and a firstinductive element connected in parallel with said measuring condenser,in which said second circuit means comprises a second network includingsaid compensating condenser and a second induc- 2,581,085 12 tiveelement connected in parallel with said comprises comparison meansresponsive to the compensating condenser, and in which said relativeimpedances of said nrst and said second indicating means comprisescomparison means electrical branches as a measure of the amountresponsive to the relative impedances of said first 01 said liquid. andsecond networks as a measure of the amount 5 ABRAHAM EDELMAN. of saidliquid.

16. Apparatus according to claim 12, wherein said power supply means isa source or alternat- REFERENCES CITED ing current, and comprising inaddition, a first The following references are oi record in theinductive element connected in said first electri- 10 m or thi patent:

cal branch in parallel with said first condenser and having a reactancevalue at the frequency of UNITED STATES PATENTS said alternating currentequal to the magnitude Number Name 7 Date of the reactance ofsaid firstcondenser when free .212.4 8 Bauder Jan. 1 6, 1917 of said liquid, and asecond inductive element 15 3 Freysliedt V- 194 connected in said secondelectrical branch in 57,023 Reid et al. Aug. 29. 1 44 parallel with saidsecond condenser and having 3,377,275 Smith M y 29, 1945 a reactancevalue at the frequency of said alter- FOREIGN PA-I'ENTS nating currentequal to the magnitude or the reactance of said liquid; and wherein saidcircuit 0 Number Cmmtry Date for controlling the indication of saidindicator 1,576 Great Britain 1936

